Most current processes used by the chemical and energy industries to transform chemical reactants into commercial synthesis products are thermally driven. Reactants are mixed in reactors and are heated to specific temperatures until reactor output is at a sufficient quantity or purity level to meet commercial product specifications. Often, adding catalysts to the reactor can accelerate these thermally driven processes. Reduction in processing time typically means a reduction in cost and, a large volume of product ready for the market. Typically, thermally driven processes need to have process heating temperatures that are significantly higher than the theoretical reaction temperature of the specific synthesis in order to ensure that product output meets the required quantity and quality levels. Additionally, excess process temperatures are needed in order to account for heat losses from the reactor to the atmosphere, depletion of catalyst reactivity over time and heat losses in products and by-products, including steam and exhaust gases.
Often, the products produced from thermally driven processes are a mixture of desired product and unwanted by-products, the latter sometimes as high as 50%. Undesirable chemical reactions may occur in parallel with the target reaction in part due to excessive temperatures employed to drive the reaction. Such reactions can drain some of the thermal energy input, leaving the desired reaction with insufficient thermal energy. Additionally, the product streams from such processes typically require additional separation or purification steps to obtain the desired product. To deal with such side reactions special measures are sometimes required in order to block or slow them so that the product can be produced in sufficient quantity and at acceptable quality. Separation and purification processes also typically require additional thermal energy input and contribute to exhaust heat or other types of energy losses, thereby further reducing process efficiency. Improvements in process energy efficiency are required by the chemical and energy industries in order to generate desired products and reduce or eliminate product and energy wastes. Such wastes represent lost profit opportunities, in the form of unrecovered molecules and damage to the environment, such as greenhouse gas emissions.
In particular with regard to fossil fuels, as their supply dwindles and deleterious environmental effects fossil fuel use increases, it is becoming increasingly evident that new or improved fuels and forms of energy are needed. Significant efforts have been undertaken over the years to identify acceptable substitutes for fossil fuels. The desired attributes of a new fuel or energy source include low cost, ample supply, renewability, Safety, and environmental compatibility.
The alternatives that are being explored can be divided into three broad categories: nuclear power, solar energy, and chemical fuels. In nuclear power, energy is extracted from the natural decay of radioactive elements. Although large amounts of energy are available from nuclear decay processes, the development of nuclear power has been limited because of concerns over the handling of radioactive elements and the disposal of radioactive waste. The public also worries about the possibility of runaway reactions and core meltdown during the operation of nuclear power plants.
Solar energy offers the promise of tapping the enormous energy reserves contained in the sun. The primary objective in solar energy development is the efficient collection and conversion of the energy contained in sunlight to electricity. The conversion is typically accomplished through photovoltaic devices that absorb and transform the wavelengths of light emitted by the sun. The transformation normally involves the production of electrical charge carriers via a valence band to conduction and absorption process in a semiconductor material. A desirable feature of using semiconductors to convert solar energy to electricity is the absence of pollution and the near zero maintenance requirements. Most solar energy devices are based on silicon and much research activity has been directed at optimizing the sunlight-to-electricity conversion efficiency through the development of better materials and innovative device structures. Although much progress has been made and will continue to be made in solar energy, efficiencies are currently limited to 10-15%.
Chemical fuels are a broad class of energy sources and encompass any substance capable of delivering energy through a chemical reaction. Conventional fossil fuels are included among chemical fuels and deliver energy through combustion reactions. The search for new chemical fuels is focusing on materials that combust cleanly and at less extreme conditions than gasoline and other petroleum based fuels. The objective of achieving clean burning fuels is directed at minimizing or eliminating environmentally undesirable by-products such as CO, CO2 and NOx gases. If reaction conditions less extreme than the high temperatures required in standard internal combustion engines can be found, an opportunity exists for developing simpler and lighter weight engines that run more efficiently. Much of the work on synfuels in the 1970's and 1980's focused on developing alternative chemical fuels for combustion engines. Various hydrocarbons and oxygenated hydrocarbon compounds such as methanol have been considered. Although some promising results have been obtained, no alternative has proven sufficiently successful to motivate the costly transition from the current fuels to a new fuel source.
Hydrogen is currently considered a desirable prospect for replacing or reducing our dependence on conventional fossil fuels. The strong interest in hydrogen is a consequence of its clean burning properties and abundance. When reacted with oxygen, hydrogen produces only water as a by-product. Hence, hydrogen is an environmentally friendly fuel. Hydrogen is also the most abundant element in the universe and is contained in large amounts in many chemical compounds. Hydrogen therefore is an attractive alternative fuel source.
The realization of hydrogen as a ubiquitous source of energy ultimately depends on its economic feasibility. Economically viable methods for extracting and/or recovering hydrogen from chemical feedstocks, as well as efficient means for storing, transferring, and consuming hydrogen, are needed. The most readily available chemical feedstocks for hydrogen are organic compounds, primarily hydrocarbons and oxygenated hydrocarbons. The most common methods for obtaining hydrogen from hydrocarbons and oxygenated hydrocarbons are dehydrogenation reactions and oxidation reactions. Dehydrogenation reactions produce hydrogen by transforming saturated hydrocarbons to unsaturated hydrocarbons. Reformation reactions are a common type of oxidation reaction and involve the breaking of bonds between hydrogen and other atoms such as carbon, oxygen or nitrogen. Hydrogen atoms released upon bond breakage combine to form the desired diatomic hydrogen molecules. The broken bonds remaining on the feedstock molecules recombine or reform to produce new molecules. The reformation process is formally an oxidation reaction of the feedstock molecules.
Production of hydrogen from hydrocarbon and oxygenated hydrocarbon compounds is frequently accomplished with a steam reformation process. In steam reformation processes, a hydrocarbon (e.g., methane) or oxygenated hydrocarbon (e.g. methanol) feedstock is contacted with water in a high temperature reactor to produce hydrogen gas (H2) along with carbon monoxide (CO) and/or carbon dioxide (CO2). Representative hydrogen producing steam reformation reactions for a general hydrocarbon (CnHm) and a general alcohol (CpHqOH) are given below:CnHm+xH2O(m/2+x)H2+yCO2+(n−y)COCpHqOH+rH2O([½](q+l)+r)H2+vCO2+(p−v)CO
The hydrocarbon CnHm can be an alkane, alkene or alkyne and the group CpHq on the general alcohol can be an alkyl, alkenyl, or alkynyl group. Similar reactions can be used to describe the production of hydrogen from other oxygenated hydrocarbons such as aldehydes, ketones, and ethers. The relative amounts of CO2 and CO produced depend on the specific reactant molecule, the amount of water used, and the reaction conditions (e.g. temperature and pressure).
For purposes of the present invention, the scope of the word “fuel” includes not only hydrogen, but additionally includes conventional hydrocarbon fuels such as methane and methanol, as well as chemicals that can be converted to useful fuels, including ammonia, sulfur, sulfur compounds such as hydrogen sulfide, as well as CO2 and N2, which are capable of being converted to products which are themselves fuels or intermediates for the production or synthesis of fuels. In some instances the purpose for converting one chemical compound into another may not be immediately apparent. For example, according to the processes of the present invention methane can be converted to methanol and while both are considered to be fuels, there can be advantages for producing methanol as a fuel. Although methane is an accepted fuel, its transportation to distant sites for use is difficult and expensive, typically including conversion to a liquid form at low temperature with an accompanying cost in energy. On the other methanol can be transported in ordinary tanker trucks and/or pipelines and it can be converted to methane at its destination by energy-efficient processes such as those described herein.
Although the prior art steam reformation processes effectively generate hydrogen, they suffer from several drawbacks. First, the reactions are endothermic at room temperature and therefore require heating. Temperatures of several hundred degrees are needed to realize acceptable reaction rates. These temperatures are costly to provide, impose special requirements on the materials used to construct the reactors, and limit the range of applications. Second, the required high temperatures imply that steam reformation reactions occur in the gas phase. This means that hydrogen must be recovered from a mixture of gases through some means of separation. The separation means adds cost and complexity to the reformation process and make it difficult to obtain perfectly pure hydrogen. Finally, the production of CO2 and/or CO is environmentally undesirable since both gases contribute to the greenhouse effect believed to be responsible for global warming.
An early patent in this field, Canadian Patent No. 787831 (Jun. 18, 1968), P. Grimes et al., teaches a liquid phase process for making hydrogen by reforming various oxidizable fuels. Liquid phase reforming can be conducted in various aqueous electrolytes but the reforming kinetics are more favorable in alkaline electrolytes, especially hydroxides, the preferred electrolytes. Conductive catalysts are used to promote reforming reactions by activating electrochemical pathways. Preferred catalysts are from the Group VIIIA transition metals. The following reaction describes the overall liquid phase reforming of methanol to produce hydrogen.CH3OH (liquid)+H2O (liquid)→CO2+3H2 
The patent discloses a batch process using a mixture of water, an ionic conductive electrolyte, and an organic compound (fuel) which react in the presence of an electronic conductive catalyst, oxidizing the fuel and producing hydrogen. The reactions are said to occur in the liquid phase and are believed to proceed via electrochemical pathways. Thus for convenience herein, this type of liquid phase reforming in alkaline electrolytes is also referred to as electrochemical reforming (ECR). Alcohol and a wide range of organic fuels, including biomass, are disclosed as well as the production of high-pressure hydrogen. High-pressure hydrogen production is disclosed and hydroxides are described as preferred electrolytes.
Recent patents to Cortright et al., U.S. Pat. Nos. 6,964,757, 6,699,457, and 6,964,758 and published U.S. Patent Application 20050207971, and Reichman et. al., U.S. Pat. Nos. 6,890,419 and 6,994,839 and published U.S. Patent Application 20050163704 are similar in many respects to the disclosures in Grimes et al. These include liquid phase reforming of alcohols, sugars, biomass, hydrocarbons and various oxygenated hydrocarbons to make hydrogen. These patents and published applications disclose the use of various ionic conducting electrolytes in the liquid phase and the use of conductive metal catalysts from Group VIII and related catalysts. The processes disclosed by Cortright et al., are conducted at pH<10, where the by-product carbon dioxide leaves as an impurity with the product hydrogen.
U.S. Pat. No. 6,607,707 discloses that hydrogen can be produced by combining an alcohol such as methanol with a base and further in the presence of a catalyst such as a transition metal and wherein the pH of the mixture is “at least 10.3,” but nothing specific is provided beyond that limited disclosure.
U.S. Pat. No. 6,890,419 discloses an electrochemical cell consisting of anode and cathode electrodes across which an external voltage is impressed and employing acidic to strongly basic electrolyte solutions, including the use of KOH up to 12M, in order to effect production of hydrogen.
U.S. Pat. No. 6,994,839 and published U.S. Patent Application 20050163704 further disclose that alkali hydroxide electrolytes are converted in a batch, process to less active alkali carbonate and bicarbonates and that the spent electrolyte can be regenerated using an energy intensive thermal process. However, this approach is economically unfavorable because the heat required to regenerate alkaline earth oxide/hydroxide reactants is significant and costly.
U.S. Pat. No. 6,953,873 (R. D. Cortright et al.) discloses a method of producing hydrocarbons from oxygenated hydrocarbon reactants having at least two carbons, such as glycerol, glucose, or sorbitol. The method can take place in the vapor phase or in the condensed liquid phase (preferably in the condensed liquid phase). The method includes the steps of reacting water and a water-soluble oxygenated hydrocarbon having at least two carbon atoms, in the presence of a metal-containing catalyst. The catalyst contains a metal selected from the group consisting of Group VIIIB transitional metals, alloys thereof, and mixtures thereof. These metals are supported on supports that exhibit acidity or the reaction is conducted under liquid-phase conditions at acidic pH. The disclosed method allows the production of hydrocarbon by the liquid phase reaction of water with biomass-derived oxygenated compounds.
There remains a need for simple, efficient methods for producing hydrocarbons, hydrogen and other fuels from various resources, especially renewable resources and those that currently present a disposal problem in view of their availability as products or by products from other industrial processes. Furthermore, there is a particular need to convert particular fuels that may be difficult to transport into more convenient fuel forms that are more readily transported and can be readily and efficiently re-converted to a useful fuel form where needed.